npm.bbclass: use python3 for npm config

python2-native executable is not available in sysroot anymore, which
causes compilation of some nodejs modules to fail. Switch to python3 as a
default python version.

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>